As a refrigerator, there is known a turbo refrigerator that is provided with a turbo compressor that rotates an impellor with an electric motor to compress and discharge a refrigerant. In the turbo compressor, a lubricating oil is supplied from an oil tank to the sliding portions such as the bearings of the rotating shaft of the electric motor and the bearings of the rotating shaft of the impeller. At the regions where the lubricating oil is supplied in this way, there is a possibility of oil leaking from the seal portions that seal the periphery of the rotating shaft to another space.
Conventionally, a liquid level indicator having a sight glass for observing the liquid level height in an oil tank is provided in order to confirm the occurrence of this oil leakage. The following Patent Document 1 discloses a differential pressure-type liquid level gauge as one device for liquid-level height observation. This differential pressure-type liquid level gauge has a constitution that is suited to a tank for liquefied gas, with the distal end portion, which includes an opening end, of the high-pressure side connecting tube being inserted in a horizontal manner or nearly horizontal manner into the interior portion of the tank for liquefied gas in the vicinity of the bottom portion.